1. Field of the Invention
The invention relates generally to relieving residual stress in strain hardened metals, and more particularly to a method of localizing the stress near the surface of an extruded, cold drawn section and alternatively bending the section out of alignment first relative to a horizontal longitudinal axis and then relative to a vertical longitudinal axis, thereby flexing the cold drawn section beyond its elastic limit for the purpose of relieving residual stress.
2. Statement of the Prior Art
It is well known in metallurgy that metals have improved physical properties brought about in an operation of cold drawing the metal, which has the form of a rod, bar, or the like, through a cold draw die to effect a change in the cross-sectional area of the metal form.
A metal subjected to cold working or plastic deformation, as it is sometimes referred to, is said to be "strain hardened" in response to the cold working, and it possesses physical characteristics after the plastic deformation process that are commercially desirable such as improved machinability, improved resistance to wear and improved surface finish or dimensional tolerance.
The effects of cold working or plastic deformation are not all desirable because cold working metal also results in the development of internal elastic stresses in the metal, generally referred to as residual stresses. During plastic deformation, the registry between adjacent planes of atoms is disturbed resulting in the formation of dislocations. The disturbance to the atoms and crystal lattice during plastic deformation causes an increase in the stored strain energy in the metal and residual stresses. Various techniques have been devised to relieve the residual stresses arising as a consequence of strain hardening the metal.
One mechanical treatment technique, shot peening, has been used to modify the residual stress pattern and to introduce some compressive stresses in the peened surface. For example, U.S. Pat. No. 4,084,585 discloses a prior art method for increasing the resistance to brittle fractures in metals by shot peening or surface rolling the metal workpiece to increase the fatigue strength.
A mechanical treatment limited to sheet metal has been used in U.S. Pat. No. 1,271,703 which discloses a method of eliminating strain marks from sheet metal articles by transversely flexing the metal using parallel rolls of a straightening apparatus. U.S. Pat. Nos. 1,649,704 and 1,649,705 disclose another apparatus for removing surface irregularities in sheet metal by first working the sheet between one set of cooperating rolls positioned to flex and bend the sheet of metal and then deflecting the sheet by an auxiliary bending roll to give the sheet an initial bending greater than that imparted to it by the set of cooperating rolls.
Another mechanical treatment for circular section steel bars used as concrete structural members, has been to roll the bars through between circular plates. As disclosed in U.S. Pat. No. 3,031,750, the bar is cold worked by bending successive portions of the bar transversely back and forth and simultaneously rotating the bar about its longitudinal axis.
The most common method of stress relief is stress relieving annealing, which requires heating the metal to a temperature below the recrystallization temperature of the metal for a period of time wherein readjustment of the crystal structure relaxes the residual stresses. An undesirable side effect of annealing is oxidation on the surface of the metal which is commonly removed by dipping the metal in a diluted sulfuric acid pickling solution having sodium dichromate. From an environmental standpoint, the chromic acid dip is very undesirable because it pollutes the water with sodium dichromate which is a very toxic substance.
Another problem with the prior art devices, such as mechanical straighteners, which advertise a reduction of residual stress, is that the actual reduction in residual stress is insufficient. A standard test for residual stress of free cutting brass is the mercurious nitrate test, ASTM B 154 wherein the cold draw extrusion is immersed in a solution of mercurious nitrate and nitric acid following degreasing, pickling, etc. The residual stress in the rod becomes visible in a series of cracks or crazing. A hot extruded cold draw part that is mechanically straightened and tested using the mercurious nitrate test will still show a series of stress cracks flowing in the direction of the cold draw. Thus, conventional mechanical straightening does not completely remove the residual stress.
The various heat and mechanical treatments taught by the prior art are unacceptable in terms of equipment cost, labor, environmental impact, and actual stress relief. The method of the present invention solves substantially all of the problems of the prior art by providing a continuous process that relieves residual stress using a unique mechanical treatment.